This invention relates to a method and an apparatus for detecting metals and is especially but not exclusively suited to monitoring metallic debris in fluids generally and in lubricating oil in particular. In other examples the invention can be made to respond to larger metallic items such as coins.
In many engineering applications, moving parts in contact with each other must be lubricated. Despite the efficiency of the lubrication method employed, over a period of time, wear inevitably occurs. The result is that small particles of the material from which the parts concerned are made are released and carried away in the moving oil. It follows that measurement of the levels and type of particulate contamination in a lubricating oil can be used as an indicator of the mechanical condition of the equipment.
While it is a relatively simple task to perform off-line analysis of oil samples, continuous monitoring of an oil in situ presents a more difficult problem, requiring appropriate transducer systems measuring an appropriate property which changes with the build up of contamination of the oil.
One form of apparatus which has been proposed for the continuous monitoring cf particulate contamination of lubricating oil is described in British Patent Specification No. 1 348 881 and uses a bridge circuit to sense the differential changes in impedance of two coils magnetically coupled to the flowing oil due to particles carried along by the oil. British Patent Specification No. 1 510 103 and British Patent Application No. 2 004 374A utilize the change in resonant frequency of a tuned circuit due to the passage of a metallic particle through the inductor of the circuit to provide an indication of the presence of the particle.
It is also known from British Patent Specification No. 1 510 103 that ferromagnetic, electrically conductive and electrically resistive particles produce different effects on the inductance of a coil when they are magnetically coupled to it.